This invention pertains generally to antennas for radio frequency energy and particularly to antennas required to produce electromagnetic beams over wide angles of coverage volume.
It has been suggested that a so-called "wide angle scanning array antenna" assembly, as described in U.S. Pat. No. 3,755,815, may be used when it is desired to deflect a radar beam through a deflection angle which may be greater, in any direction, than the maximum feasible deflection angle of a beam from a conventional phased array. Briefly, such an antenna assembly consists of a conventional planar phased array mounted within a structure which acts as a lens. When any portion of such a structure is illuminated in a controlled fashion by a radar beam from the planar phased array, the direction of such radar beam with respect to the boresight line of the planar phased array is changed in a manner analogous to the way in which a prism bends visible light. Thus, the deflection angle of the radar beam propagated in free space may be caused to be much larger than the greatest deflection angle attainable with a planar phased array.
Although an antenna assembly made in accordance with the disclosure of the cited patent is, in theory, suited to the purpose of deflecting a radar beam through extremely wide deflection angles without requiring any rotary joints for the radar energy or mechanical movement of the entire assembly, its complexity militates against its use in many applications.
The complexity of the arrangement described in the patent referred to above is caused mainly by the fact that, for each different position of the beam, the shape of the wavefront of the beam from the planar array must be changed. While the requisite change in shape of the wavefront of the beam from the planar array may be accomplished by controlling the individual antenna elements in such array by signals from a computer, the capacity of the computer must be relatively large if sufficiently precise control of the shape of the wavefront is to be attained to achieve satisfactory collimation of the beam finally propagated in free space.
Another difficulty with the type of antennas disclosed in the cited patent resides in the fact that, when any radar beam is deflected from the normal to the plane of a planar phased array, degradation in beam shape is experienced. Such degradation, which may be deemed to be the result of foreshortening of the aperture of the planar phased array as the deflection angle is increased, limits the maximum feasible deflection angle from such an array to between 45.degree. to 60.degree.. If, then, a deflection angle greater than 90.degree. is required for the beam in free space, it is necessary that the lens structure be effective to cause a relatively large change in beam direction. That is to say, the lens structure must be equivalent to a "thick" lens with the concomitant shortcomings of such a lens.